Interpretive criteria of ceftibuten disk diffusion susceptibility tests according to the DIN 58 940 method

Objective   This study aimed to establish interpretive criteria for agar diffusion tests with ceftibuten disks according to DIN standards.
Methods   Minimal inhibitory concentrations (MICs) and inhibition zones produced by ceftibuten in the disk diffusion test were determined for 275 recent bacterial isolates, including 11 species with 25 strains each. Regression analysis was performed for two disk loads (10 µg and 30 µg).
Results   Correlation of MICs and zone diameters was good, with correlation coefficients of r  = ? 0.97 for both tested disk loads. Evaluation of the calculated zone size criteria for all species showed no very major discrepancies or no major discrepancies. The 30-µg disks, however, produced unacceptably large inhibition zones for very susceptible strains, so that usage of 10-µg disks must be recommended when testing according to DIN standards.
Conclusion   Based on the MIC breakpoints recommended by the DIN (≥8 mg/L and ≤ 1 mg/L), the following interpretive breakpoints for disk diffusion susceptibility tests with 10-µg ceftibuten disks were calculated using regression line analysis: ≤19 mm for resistance and ≥ 27 mm for susceptiblity. Proposed inhibition zone diameters for the reference strain Escherichia coli ATCC 25922 are between 31 and 36 mm.